Re: [PATCH v4 tip/perf/core 2/4] mm: Introduce mmap_lock_speculation_{begin|end}

[Vlastimil Babka]

On 10/28/24 02:08, Andrii Nakryiko wrote:
> From: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> 
> Add helper functions to speculatively perform operations without
> read-locking mmap_lock, expecting that mmap_lock will not be
> write-locked and mm is not modified from under us.
> 
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>  include/linux/mmap_lock.h | 29 +++++++++++++++++++++++++++--
>  1 file changed, 27 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> index 6b3272686860..58dde2e35f7e 100644
> --- a/include/linux/mmap_lock.h
> +++ b/include/linux/mmap_lock.h
> @@ -71,6 +71,7 @@ static inline void mmap_assert_write_locked(const struct mm_struct *mm)
>  }
>  
>  #ifdef CONFIG_PER_VMA_LOCK
> +
>  static inline void mm_lock_seqcount_init(struct mm_struct *mm)
>  {
>  	seqcount_init(&mm->mm_lock_seq);
> @@ -86,11 +87,35 @@ static inline void mm_lock_seqcount_end(struct mm_struct *mm)
>  	do_raw_write_seqcount_end(&mm->mm_lock_seq);
>  }
>  
> -#else
> +static inline bool mmap_lock_speculation_begin(struct mm_struct *mm, unsigned int *seq)
> +{
> +	*seq = raw_read_seqcount(&mm->mm_lock_seq);
> +	/* Allow speculation if mmap_lock is not write-locked */
> +	return (*seq & 1) == 0;
> +}
> +
> +static inline bool mmap_lock_speculation_end(struct mm_struct *mm, unsigned int seq)
> +{
> +	return !do_read_seqcount_retry(&mm->mm_lock_seq, seq);
> +}
> +
> +#else /* CONFIG_PER_VMA_LOCK */
> +
>  static inline void mm_lock_seqcount_init(struct mm_struct *mm) {}
>  static inline void mm_lock_seqcount_begin(struct mm_struct *mm) {}
>  static inline void mm_lock_seqcount_end(struct mm_struct *mm) {}
> -#endif
> +
> +static inline bool mmap_lock_speculation_begin(struct mm_struct *mm, unsigned int *seq)
> +{
> +	return false;
> +}
> +
> +static inline bool mmap_lock_speculation_end(struct mm_struct *mm, unsigned int seq)
> +{
> +	return false;
> +}
> +
> +#endif /* CONFIG_PER_VMA_LOCK */
>  
>  static inline void mmap_init_lock(struct mm_struct *mm)
>  {